Washington – Tuesday, the last big primary election day this year, could be the biggest test yet of tea party influence, as the conservative grass-roots movement appears within striking distance of denying veteran moderate U.S. Rep. Michael Castle the Republican U.S. Senate nomination in Delaware.

So far this year, tea party activists have turned voter anger against Washington into a purge of Republicans they regard as too cozy with Democrats and toppled GOP establishment Senate favorites around the country.

The insurgents are on the move again in Delaware. Defeating Castle would be their biggest coup yet, a loss that would rock the political world seven weeks before November’s congressional and gubernatorial elections.

Castle has served Delaware since 1966 as a state legislator, lieutenant governor, governor and, since 1993, the state’s only congressman, usually winning his dozen statewide races by huge margins. The state Republican Party establishment is aggressively boosting his Senate candidacy to fill the seat held until 2009 by Vice President Joe Biden and since then by Democrat Ted Kaufman.

A poll last weekend found Castle slightly behind media consultant Christine O’Donnell, a perennial candidate who was barely known even a month ago and whose personal financial record is a patchwork of debts and delinquent taxes. Public Policy Polling, which surveyed 668 likely Republican voters Saturday and Sunday, found the race too close to call, with O’Donnell ahead of Castle by 3 percentage points. The poll had an error margin of 3.8 percentage points.

O’Donnell, who resembles former Alaska Gov. Sarah Palin in appearance and speaking style, has fired up the conservative base with support from Palin, the California-based Tea Party Express, which has pledged $250,000 to her campaign, and Sen. Jim DeMint, R-S.C.

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If Castle lost, he’d be the latest in a growing list of mainstream GOP candidates toppled by tea party challenges. Two were incumbent senators: Utah’s Robert Bennett, who lost at a party convention, and Alaska’s Lisa Murkowski, whose primary defeat was due partly to Palin’s opposition in her home state. Other unsuccessful GOP establishment figures in Kentucky, Colorado, Nevada and Florida had strong backing from their party leaders for Senate nominations but were overthrown or pushed aside by tea party insurgents.

A Castle loss would be arguably the biggest upset yet, because his centrist record and genial style have made him one of the most popular figures for decades in Delaware, a tiny state where he’s known personally in every hamlet. However, being a genial centrist appears to be a liability in this year’s Republican primaries, when grass-roots activists want to elect conservative revolutionaries.

O’Donnell has momentum.

“There’s some evidence it (the race) is shifting, that Castle is in trouble and the race is a dead heat,” said Joseph Pika, a professor of political science at the University of Delaware.

He said that O’Donnell’s backers, “fanned by the national voices who have chimed in, are the more motivated to vote and could surprise everyone.”

Castle and the GOP establishment are battling back. On Monday, the state GOP sent reporters a list of what it called “more troubling stories about O’Donnell’s past.”

Earlier this year, the Internal Revenue Service placed a lien against O’Donnell for not paying taxes; she says the IRS later conceded that it had made an error. Dean Patterson, an IRS spokesman, said Monday, “We are unable to discuss individual tax matters.”

The Wilmington News Journal has documented O’Donnell’s near-foreclosure on her mortgage and a dispute with her alma mater over payment of college expenses.

Analysts see O’Donnell benefiting from the same anger that’s triggered other Republican insurgencies.

“This is a revolution of the haves,” said Ross Baker, a professor of political science at Rutgers University in New Jersey. “Most of these tea party people are not poor people, but they are angry and worried.”

Many have never accepted President Barack Obama.

“They’ve had questions about his legitimacy all along,” Baker said.

They’re also concerned that government keeps growing and running deeper debt.

Two other states also will be watched Tuesday for evidence of tea party influence in Republican primaries.

In New Hampshire, former Attorney General Kelly Ayotte had been a strong favorite for the GOP Senate nomination, but polls now show lawyer Ovide Lamontagne gaining; he’s attracted some tea party support. While Ayotte is seen as the establishment choice, she’s also won backing from Palin, who calls her “one tough ‘Granite Grizzly.’ “

Lamontagne is appealing as an outsider, said Andrew Smith, the director of the University of New Hampshire Survey Center. “Lamontagne looks like Ronald Reagan. He exudes enthusiasm,” Smith said. DeMint also backs him.

In Maryland, little-known investor Brian Murphy is challenging former Gov. Robert Ehrlich — who was elected the state’s first Republican governor since 1966 in 2002, but lost in 2006 — for the GOP gubernatorial nomination. Palin endorsed Murphy this summer, but has done little to help him.

Also holding primaries Tuesday are Wisconsin, Rhode Island, New York state, Massachusetts and the District of Columbia, where City Council Chairman Vincent Gray is challenging incumbent Democratic Mayor Adrian Fenty. Gray leads in polls in the heavily Democratic city, where winning the primary usually means a general election victory.

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